A flow meter device measures the flow (rate) of a fluid flowing through a measurement fluid passage. In this flow meter device, the flow velocity of the fluid is compensated (modified) to obtain an average flow velocity in the entire measurement fluid passage by multiplying the measured flow velocity of the fluid by a compensation (modification) coefficient, and the flow (rate) is calculated based on the compensated flow velocity and the cross-sectional area of the measurement fluid passage.
However, the compensation coefficient cannot be set uniquely due to an influence of the configuration of the flow meter device, etc. If setting of the configuration or the like of the flow meter device is changed, then it becomes necessary to newly set the compensation coefficient. There is known a flow coefficient setting method which is intended to automatically set a flow coefficient (compensation coefficient) easily using a personal computer or the like (see e.g., Patent Literature 1).
Hereinafter, the flow coefficient setting method disclosed in Patent Literature 1 will be described with reference to FIG. 10. FIG. 10 is a graph showing a flow velocity (V), a flow coefficient (K), and an approximate line thereof, disclosed in Patent Literature 1.
As shown in FIG. 10, in the flow coefficient setting method disclosed in Patent Literature 1, adjacent data set of an arbitrary number (set of the flow velocity measured by a flow velocity measurement means and a flow coefficient corresponding to the flow velocity), is selected, and an approximate line is calculated using the selected data set.
Specifically, based on measurement points (adjacent data set) 100 to 104 within a region A, an approximate line 10 optimized by a least square method or the like is found. Then, the value of the flow coefficient derived using the approximate line 105 is compared to the value of the flow coefficient derived from the measurement value to determine whether a difference between these values set falls within a predetermined error range. If the difference falls within the predetermined error range, new data set is added, and the range which can be approximated as the approximate line is sequentially derived by the same procedure.
In the flow coefficient setting method disclosed in Patent Literature 1, the flow velocity measured by the flow velocity measurement means and the flow coefficient corresponding to the flow velocity are plotted to calculate an optimal approximate function, and the flow coefficient corresponding to the flow velocity measured by the flow velocity measurement means is calculated using the optimal approximate function.    Patent Literature 1: Japanese Patent Publication No. 3487589